1. Field of the Invention
The present invention relaters to a semiconductor optical sensor, and more particularly, to a semiconductor optical sensor for detecting ultraviolet radiation.
2. Description of the Related Art
In a conventional semiconductor optical sensor, an N+ diffusion layer formed as an E-shaped comb and in which an n-type impurity is diffused to a high concentration and a P+ diffusion layer formed as a Π-shaped comb and in which a p-type impurity is diffused to a high concentration are arranged horizontally, opposing each other and with the comb-tooth portions of thereof meshing into one another, on a silicon semiconductor layer having diffused therein an n-type impurity to a low concentration, in an SOI (silicon on insulator) substrate having formed therein silicon semiconductor layers of a thickness of about 150 nm sandwiching an embedded oxide layer, on a silicon substrate, in such a way that ultraviolet radiation intensity is detected by applying a predetermined voltage to metallic wiring electrically connected to the N+ diffusion layer and the P+ diffusion layer. See, for instance, Japanese Unexamined Patent Publication Kokai No. H07-162024 (hereinafter, also referred to as a patent document-1).
Conventional photodiodes, for their part, have a light-shielding film that covers virtually the entire surface of a semiconductor chip, excluding a photodiode opening portion. Between a photodiode and circuit elements formed on a silicon substrate there is also arranged a dummy photodiode and/or a cathode electrode. Such a constitution prevents external light from being incident on circuit elements, thereby preventing malfunction of the circuit elements. See, for instance, Japanese Unexamined Patent Publication Kokai No. H09-293847 (hereinafter, also referred to as a patent document-2).
In a semiconductor optical sensor using an SOI substrate, however, the silicon layer is thin, of several tens of nm, and absorbs thus hardly any ultraviolet light or visible light; as a result, part of the ultraviolet light or visible light reaches the embedded oxide film. Herein, ultraviolet light denotes light having a wavelength of 400 nm or less. Meanwhile, ultraviolet light or visible light is hardly absorbed by the embedded oxide film. Also, part of the ultraviolet light or visible light is reflected at the interface between the embedded oxide film and/or at the interface between the embedded oxide film and the silicon layer. As a result, part of the ultraviolet light or visible light is not absorbed and becomes repeatedly reflected, propagating within the embedded oxide film and reaching the circuit elements, which may give rise to circuit element malfunction.
Propagation of ultraviolet light or visible light within embedded oxide films needs thus to be addressed in semiconductor optical sensors using SOI substrates, and hence simply arranging a light-shielding film provided on a interlayer insulating film, arranging a dummy photodiode in a substrate, or arranging a cathode electrode formed on the substrate, as disclosed in the patent document-2, was not sufficient for preventing incident light from penetrating into the circuit elements.